Аннотация
When investigating topographically irregular layers in the near surface
with shear waves, it is of particular importance to consider the
3D-nature of wave propagation. Depending on the layer geometry and
on the spatial arrangement of source- and receiver-points significant
lateral ray bending can occur causing side-swipe traveltime effects
and complicated polarisation patterns. As an example we present a
study where 3D-shear wave refraction measurements were applied in
order to reconstruct the geometry of a silted ancient harbour basin
at the archaeological site of Miletus (West Turkey). Seismic signals
were generated with a three-component vector force and recorded with
three-component geophones arranged in 2D-arrays of 1 m grid spacing.
Since a correct identification of refracted S-wave arrivals is a
precondition to traveltime interpretation we investigated a method
to decompose these wavefields with respect to their polarisation
and azimuth of propagation. Taking advantage of the 2D-geophone arrangement
we applied the following processing approach: In case of general
lateral heterogeneity a decomposition can be performed by applying
the curl and divergence operations to the vector wavefields recorded
in 2D-arrays. The separated tangential and normal components to the
wavefront in a plane are finally enhanced by combining the different
force components in order to eliminate the radiation characteristics
of the source. The decomposed wavefield was then the basis for 3D-refractor
imaging through a newly formulated map migration of the refracted
traveltime field. This technique was developed to map coherent basement
structure on the meter-scale. Supplemental tomographic inversion
using the refractor topography model as input provided a plausible
velocity model, exhibiting characteristic anomalies such as a prominent
low velocity zone overlain by a high velocity layer in the refractor.
The seismic velocity structure suggests that the harbour basin was
locally filled with natural or anthropogenic debris on which an antique
harbour wall could have been founded.
Пользователи данного ресурса
Пожалуйста,
войдите в систему, чтобы принять участие в дискуссии (добавить собственные рецензию, или комментарий)